He's Luke's rival (maybe by those who think Lucas making Leia Luke's sister was a mistake).
He's a pirate and a smuggler of a rather addictive drug with nasty side-effects (maybe by EU fans)
He's disturbingly pushy with Leia and doesn't seem to get that no means no (maybe by those who know just how creepy that is)
He's boastful.
He's self-centred.

Han Solo as well. Not to say that I dislike him, necessarily, but he's a character I outgrew my fascination with as soon as I was in grade school. He's a wish fulfillment character, and I can understand if you want to just latch on to him as an object of fun and adventure, but there are just so many other characters in the films that hit me on a deeper level, and with whom I was able to identify with a hell of a lot more (Luke and Anakin, especially). Besides, if I really want to watch Harrison Ford doing his thing and being a wish fulfillment hero, I'd rather put on an Indiana Jones movie.

Can't think of any to hate but I've been an avid fan since I was three. Palpatine is evil so he is my least favourite character but at the same time he is vital to the story. Maybe people should think about that.

Jar Jar was needed, in writers terms since I'm a fan of the clumsy Gungan, to pretty much end the Republic for Palpatine's plans. He was needed, who else could do it?